The human nervous system

The human nervous system is extremely complex, yet we rely on it every day to help us stay safe and
healthy.

The nervous system is the body’s inner communication system and is made up of the body’s many nerve cells.

It is these nerve cells that take information from the body’s five senses: touch, taste, smell, sound and sight! The brain is then able to react to these senses and interact with the environment.

The human nervous system is made up of the central nervous system (CNS) which is the brain and the spinal cord, and the peripheral nervous system which is made from nerve cells that carry information to and from the CNS.

When a sensory receptor detects a stimulus, the information is passed along neurones. Neurones are a special type of cell. They are found in groups/bundles of many hundreds of neurones known as nerves.

Looking in a little more detail we can see how the nervous system works: A stimulus is detected by a sensory receptor in the body. The receptor sends this information (via electrical impulses) along sensory neurones to the central nervous system (CNS). It is these sensory neurones which are responsible for converting an external stimuli into an internal stimuli.

The CNS then coordinates the information and sends another electrical impulse (this time along the motor neurones) to the effector organs. These then carry out the response action to the stimulus.

If the brain is involved in this process, the overall feedback will be voluntary. If only the spinal cord is involved, then the feedback is involuntary or a reflex (a reflex arc).

In a reflex arc, the impulse travels from a receptor, down the sensory neurone, through the white matter and into the grey matter. Here, the impulse crosses a synapse (explained further down) onto a relay neurone, still in the grey matter. The relay neurone then passes the impulse to a motor neurone, again across a synapse. Finally, the motor neurone carries the impulse to an effector, which carries out the relevant reflex response.

Motor neurons receive impulses in the spinal cord and transmit them to effectors to carry out the desired response.

The junctions between neurones are called synapses. Neurones are not joined directly to one another. Instead, there are junctions between them that are called synapses. Any electrical impulses that are travelling along the neurones have to cross the synapses (they cannot just jump the gap).

Reflex actions often occur very rapidly and are carried out to prevent direct damage to your bod, for example not scalding yourself on a boiling pan. Reflexes are usually not things you do consciously, for example when you are chewing food. This is because you could choose not to chew if you wanted to and therefore it is not an example of a reflex action.

Let’s take a look at some good examples of reflexes and how they protect the body from harm:

Our eyes. In bright conditions, the circular muscles contract, causing the pupil to reduce in size, thus reducing the intensity of light entering the eye. In dim conditions, the radial muscles contract, causing the opposite effect.

This is down to the retina of the eye being light sensitive. The retina sits at the back of the eyeball. It detects light energy and converts this into an electrical impulse to send to the brain via the optic nerve.

The cornea is the transparent front part of the eye. The lens refracts the light after it has travelled through the cornea. The ciliary muscles are involved in controlling how convex the shape of the lens is, while the retina contains light sensitive cells that pass electrical impulses along the optic nerve to the brain.

To refract light from a distant object, the ciliary muscles relax, which tightens the suspensory ligaments and causes the lens to be less rounded. To refract light from a nearby object, the ciliary muscles contract, which loosens the suspensory ligaments and causes the lens to be more rounded.

Our skin receptors also detect stimuli and send information to the CNS. The fat layer is for insulation and the hair erector muscles control whether the hair follicles stand erect or lie flat.

The skin receptors detect various stimuli such as heat, pressure and pain and send impulses to the central nervous system to respond accordingly.

Sweat glands produces sweat to cool your body down, while the capillaries bring blood and vital nutrients to the cells that make up your skin, as well as heat energy to be lost by radiation if your body temperature is too high.

So you see even when we’re not thinking hard about what we’re doing, our body is working on its own system to keep us safe 24/7!